A variety of electronic circuits are used to drive diode loads and, more particularly, to control electrical current through strings of series connected light-emitting diodes (LEDs), which, in some embodiments, form an LED display, or, more particularly, a backlight for a display, for example, a liquid crystal display (LCD). It is known that individual LEDs have a variation in forward voltage drop from unit to unit. Therefore, the strings of series connected LEDs can have a variation in forward voltage drop.
Strings of series connected LEDs can be coupled to a common switching regulator, e.g., a boost switching regulator, at one end of the LED strings, the switching regulator configured to provide a high enough voltage to supply each of the strings of LEDs. The other end of each of the strings of series connected LEDs can be coupled to a respective current sink, configured to sink a relatively constant current through each of the strings of series connected LEDs.
It will be appreciated that the voltage generated by the common switching regulator must be a high enough voltage to supply the one series connected string of LEDs having the greatest total voltage drop, plus an overhead voltage needed by the respective current sink. In other words, if four series connected strings of LEDs have voltage drops of 30V, 30V, 30V, and 31 volts, and each respective current sink requires at least one volt in order to operate, then the common boost switching regulator must supply at least 32 volts.
While it is possible to provide a fixed voltage switching regulator that can supply enough voltage for all possible series strings of LEDs, such a switching regulator would generate unnecessarily high power dissipation when driving strings of series connected LEDs having less voltage drop. Therefore, in some LED driver circuits, the voltage drops through each of the strings of series connected LEDs are sensed (for example, by a so-called “minimum select circuit”) to select a lowest voltage appearing at the end of one of the strings of series connected LEDs and the common switching regulator is controlled to generate an output voltage only high enough to drive the series connected LED string having the lowest voltage (i.e., the highest voltage drop). One such minimum select circuit is described, for example, in U.S. Pat. No. 6,822,403.